Question: Which of these numbers is composite? ${2,\ 23,\ 31,\ 39,\ 61}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 23, 31, and 61 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.